Re: KASAN: use-after-free Read in __do_page_fault

[Vlastimil Babka <vbabka@suse.cz>]

On 10/30/2017 08:15 PM, Dmitry Vyukov wrote:
> On Mon, Oct 30, 2017 at 10:12 PM, syzbot
> <bot+6a5269ce759a7bb12754ed9622076dc93f65a1f6@syzkaller.appspotmail.com>
> wrote:
>> Hello,
>>
>> syzkaller hit the following crash on
>> 887c8ba753fbe809ba93fa3cfd0cc46db18d37d4
>> git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/master
>> compiler: gcc (GCC) 7.1.1 20170620
>> .config is attached
>> Raw console output is attached.
>>
>> syzkaller reproducer is attached. See https://goo.gl/kgGztJ
>> for information about syzkaller reproducers
>>
>>
>> BUG: KASAN: use-after-free in arch_local_irq_enable
>> arch/x86/include/asm/paravirt.h:787 [inline]
>> BUG: KASAN: use-after-free in __do_page_fault+0xc03/0xd60
>> arch/x86/mm/fault.c:1357
>> Read of size 8 at addr ffff8801cbfd3090 by task syz-executor7/3660

Why would local_irq_enable() touch a vma object? Is the stack unwinder
confused or what?
arch/x86/mm/fault.c:1357 means the "else" path of if (user_mode(regs)),
but the page fault's RIP is userspace? Strange.

>> CPU: 1 PID: 3660 Comm: syz-executor7 Not tainted 4.14.0-rc3+ #23
>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
>> Google 01/01/2011
>> Call Trace:
>>  __dump_stack lib/dump_stack.c:16 [inline]
>>  dump_stack+0x194/0x257 lib/dump_stack.c:52
>>  print_address_description+0x73/0x250 mm/kasan/report.c:252
>>  kasan_report_error mm/kasan/report.c:351 [inline]
>>  kasan_report+0x25b/0x340 mm/kasan/report.c:409
>>  __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:430
>>  arch_local_irq_enable arch/x86/include/asm/paravirt.h:787 [inline]
>>  __do_page_fault+0xc03/0xd60 arch/x86/mm/fault.c:1357
>>  do_page_fault+0xee/0x720 arch/x86/mm/fault.c:1520
>>  page_fault+0x22/0x30 arch/x86/entry/entry_64.S:1066
>> RIP: 0023:0x8073f4f
>> RSP: 002b:00000000f7f89bd0 EFLAGS: 00010202
>> RAX: 00000000f7f89c8c RBX: 0000000000000400 RCX: 000000000000000e
>> RDX: 00000000f7f8aa88 RSI: 0000000020012fe0 RDI: 00000000f7f89c8c
>> RBP: 0000000008128000 R08: 0000000000000000 R09: 0000000000000000
>> R10: 0000000000000000 R11: 0000000000000292 R12: 0000000000000000
>> R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
>>
>> Allocated by task 3660:
>>  save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:59
>>  save_stack+0x43/0xd0 mm/kasan/kasan.c:447
>>  set_track mm/kasan/kasan.c:459 [inline]
>>  kasan_kmalloc+0xad/0xe0 mm/kasan/kasan.c:551
>>  kasan_slab_alloc+0x12/0x20 mm/kasan/kasan.c:489
>>  kmem_cache_alloc+0x12e/0x760 mm/slab.c:3561
>>  kmem_cache_zalloc include/linux/slab.h:656 [inline]
>>  mmap_region+0x7ee/0x15a0 mm/mmap.c:1658
>>  do_mmap+0x6a1/0xd50 mm/mmap.c:1468
>>  do_mmap_pgoff include/linux/mm.h:2150 [inline]
>>  vm_mmap_pgoff+0x1de/0x280 mm/util.c:333
>>  SYSC_mmap_pgoff mm/mmap.c:1518 [inline]
>>  SyS_mmap_pgoff+0x23b/0x5f0 mm/mmap.c:1476
>>  do_syscall_32_irqs_on arch/x86/entry/common.c:329 [inline]
>>  do_fast_syscall_32+0x3f2/0xf05 arch/x86/entry/common.c:391
>>  entry_SYSENTER_compat+0x51/0x60 arch/x86/entry/entry_64_compat.S:124
>>
>> Freed by task 3667:
>>  save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:59
>>  save_stack+0x43/0xd0 mm/kasan/kasan.c:447
>>  set_track mm/kasan/kasan.c:459 [inline]
>>  kasan_slab_free+0x71/0xc0 mm/kasan/kasan.c:524
>>  __cache_free mm/slab.c:3503 [inline]
>>  kmem_cache_free+0x77/0x280 mm/slab.c:3763
>>  remove_vma+0x162/0x1b0 mm/mmap.c:176
>>  remove_vma_list mm/mmap.c:2475 [inline]
>>  do_munmap+0x82a/0xdf0 mm/mmap.c:2714
>>  mmap_region+0x59e/0x15a0 mm/mmap.c:1631
>>  do_mmap+0x6a1/0xd50 mm/mmap.c:1468
>>  do_mmap_pgoff include/linux/mm.h:2150 [inline]
>>  vm_mmap_pgoff+0x1de/0x280 mm/util.c:333
>>  SYSC_mmap_pgoff mm/mmap.c:1518 [inline]
>>  SyS_mmap_pgoff+0x23b/0x5f0 mm/mmap.c:1476
>>  do_syscall_32_irqs_on arch/x86/entry/common.c:329 [inline]
>>  do_fast_syscall_32+0x3f2/0xf05 arch/x86/entry/common.c:391
>>  entry_SYSENTER_compat+0x51/0x60 arch/x86/entry/entry_64_compat.S:124

This would mean that mmap_sem is not doing its job and we raced with a
vma removal. Or the rbtree is broken and contains a vma that has been
freed. Hmm, or the vmacache is broken? You could try removing the 3
lines starting with vmacache_find() in find_vma().

>> The buggy address belongs to the object at ffff8801cbfd3040
>>  which belongs to the cache vm_area_struct of size 200
>> The buggy address is located 80 bytes inside of
>>  200-byte region [ffff8801cbfd3040, ffff8801cbfd3108)

My vm_area_struct is 192 bytes, could be your layout is different due to
.config. At offset 80 I have vma->vm_flags. That is checked by
__do_page_fault(), but only after vma->vm_start (offset 0). Of course,
reordering is possible.

>> The buggy address belongs to the page:
>> page:ffffea00072ff4c0 count:1 mapcount:0 mapping:ffff8801cbfd3040 index:0x0
>> flags: 0x200000000000100(slab)
>> raw: 0200000000000100 ffff8801cbfd3040 0000000000000000 000000010000000f
>> raw: ffffea000730c7a0 ffffea00072ff7a0 ffff8801dae069c0 0000000000000000
>> page dumped because: kasan: bad access detected
>>
>> Memory state around the buggy address:
>>  ffff8801cbfd2f80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>>  ffff8801cbfd3000: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
>>>
>>> ffff8801cbfd3080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>>
>>                          ^
>>  ffff8801cbfd3100: fb fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb
>>  ffff8801cbfd3180: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>> ==================================================================
> 
> 
> I guess this is more related to mm rather than x86, so +mm maintainers.
> This continues to happen, in particular on upstream
> 781402340475144bb360e32bb7437fa4b84cadc3 (Oct 28).
> 
> 
>> ---
>> This bug is generated by a dumb bot. It may contain errors.
>> See https://goo.gl/tpsmEJ for details.
>> Direct all questions to syzkaller@googlegroups.com.
>>
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>> Once a fix for this bug is committed, please reply to this email with:
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